Ground speed meter



S. DOBA, JR

yGROUND SPEED METER Filed March 2l, 1944 Aug. 27, 1946.

5 Sheets-Sheet 1 ...EE wuz;

. I.. wenn /N VEN TOR BY S. 008A, JR.

AGENT Aug. 27, 946.

S. DOBA, JR

GROUND lSPEED METER Filed March 21, 1944 5 Sheets-Sheet 2 v .mi mQ /NVE/vrof? S. 008A, JR.

AGENTv Aug. 27, 1946..V s, DQBA, JR 2,495,358

GROUND SPEED METER Filed March 21, 1944 5 Sheets-Sheet 3 AGENT' /N VEN TOR S. DOB/I, JR. A

ug. 27, 1946. s. DoBA, JR 2,406,353

GROUND SPEED METER 1 Filed March 21, 1944 s sheets-sheet 4 AGE/vr A s. DoBA, JR

GROUND SPEED METER Aug. 27, 1946.

v Filed March 21, m44 5 sheets-sheet 5 VvE/WOR I DOBA, JR

Patented Aug. 27, 1946 GROUND SPEED METER Stephen Doha, Jr., Long llsland City, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application March 21, 1944, Serial No. 527,459

(Cl. Z50-1) 4 Claims.

This invention relates to an improved method and apparatus for measuring the relative speed of an observer and an observed object visible or invisible. The invention while particularly useful in a military airplane ying to-ward a target ahead, has a field of use including all cases of relative movement in the air, at sea or on land- The general object of the invention, therefore, is to provide a method and means for measuring the relative speed at which an observer approaches an observed position.

The invention makes use of known electrical object locating and ranging means which are independent of weather and light, making such means an element in a novel system of apparatus for measuring the rate of change of range of a selected object. The invention thus achieves another object, namely, to provide speed measuring means useful in all conditions of observation.

In bombing an enemy target from an airplane, it is obviously important to know the relative speed of target and bombing plane. Hence another object of the invention is to facilitate the prosecution or war in the air, as well as to provide a navigational aid universally useful in time oi peace.

The invention enables the pilot of an airplane to measure his absolute speed with respect to a point ahead. If that point is fixed on the earths surface and its initial distance is large compared with the planes altitude, the absolute speed measured is substantially the ground speed of the plane, For example, ii the altitude is 10,000 feet and the distance on the earths surface is 50,000 feet between the point of reference and a point vertically beneath the plane, the speed measured is 98 per cent of the ground speed. For a surface craft the measurement requires no correction. In neither case does a head or a tail wind or ocean current affect the measurement of speed, Therefore, another object of the invention is to provide navigators with means for measuring ground speed regardless of wind or current.

The invention is to be understood from the following description, read with reference to the accompanying drawings in which:

Fig. l is a block schematic diagram of the major components of the invention;

Figs. 2 to 8 are circuit diagrams representing, respectively, time base generator 2d, range sweep generator El), rate sweep generator 8%, range diiferential amplifier lill, video mixing amplifier MB, video amplifier l'l and vertical sweep amplier 20G of Fig. 1; and

Figs. 9A and 9B illustrate the patterns on oscilloscope screen 2 produced when switch S is closed upward and downward, respectively.

In all figures like numerals and letters indicate like elements.

The invention will be described with reference to its use in an airplane which will be understood to be provided with the usual altimeter and airspeed meter. For simplicity, it will be assumed that the plane is flying directly, without leeway, toward a target ahead.

Referring now to Fig. 1, the radar system generally indicated by numeral l, not itself a part of the present invention but here briefly described to facilitate understanding of the complete system, serves to detect the presence of a target ahead and represent that target by a luminous spot T on screen 2 of cathode ray oscilloscope 3. The location of spot T on screen 2 corresponds as later explained to the range and bearing, at a given instant, of the target represented.

System l includes a pulse transmitting circuit 4 and a pulse receiving circuit 5 connected through duplexing unit 6 to a common antenna 'l which is preferably of the highly directive type consisting of a small polarized dipole 8 at the focus of a, parabolic reflector 9. Antenna 'i is connected by a coaxial link Ill through duplexing unit 6 to the circuits i and 5, with a rotary joint Il in link lil. The portion of link i above joint H is provided with gearing l2 through which motor i3 is enabled to rotate antenna 'l at a constant speed in the horizontal plane. Rotation of antenna I in a vertical plane may be accomplished by a like arrangement of motor and gearing which is omitted here as unnecessary to the present description. The pulse generator i4 supplies a positive square top pulse of very short duration to control radio modulator i5 to supply at a convenient repetition rate extremely short and intense pulses of radio frequency energy to antenna l by which these pulses are directively radiated into space. Duplexing unit 6, which may be an automatic transmitter-receiver switch of any known type, eiectively short-circuits the input to receiving circuit 5 while antenna i is emitting but allows free passage to circuit 5 of the low level echo received by antenna 1 from a reflecting target. The interval between successive emissions by antenna 'i is made longer than enough to include the reception of radio echoes from the most distant target to be attached.

A portion of the energy radiated by antenna 'l is intercepted and reflected, usually diffusely, by the target. A part of this reflected portion is received by antenna l and 'transformed into an electrical pulse which passes through duplexing unit 6 to radio receiver iii in circuit 5 where it is ampliiied and detected. The detected pulse is further ampliiied by video ainplier Il and is thus available to produce intensity modulation of the cathode ray beam of oscilloscope 3. Oscilloscope 3 may be of the Well-known magnetic deflection type and is not shown in detail in Fig. 1 beyond intensity grid i8, cathode i9, uorescent screen 2 and deiiecting coils HDC and VDC for horizontal and vertical beam deflection, respectively.

vShaft 2E, through which motor i3 drives gear l2, carries a pair of potentiometer wipers 2l and 2| insulated from each other and from Vshaft 2l) on which they are mounted radially opposite each other. Wipers 2i and 2i traverse potentiometer 22 fixed in the airplane. Battery 23 is connected across diametrically opposite points of potentiometer 22. The rotation with shaft 2li of wipers 2l and 2l selects a fraction of the voltage of battery 23 ranging from zero when the pointing of antenna 'l is directly ahead to a maximum when antenna l points abeam. The polarity oi the selected voltage depends on the left or right pointing of antenna l and the voltage so selected is applied to produce a current in horizontal deiiecting coil HDC of oscilloscope 3. Auxiliary means, not shown, are provided for horizontal centering of the cathode ray beam on screen when wipers 2l and 2l select zero voltage.

When the echo pulse from the reilecting target is available on grid is to produce intensity rnodulation of the cathode ray beam a luminous spot T representing the target will appear on screen 2 located vertically thereon at a position corresponding to the target range provided a vertical sweep current, synchronized with the emission of energy from antenna l, is caused to flow in vertical delecting coil VDC. The horizontal sweep current in coil HRC insures that the target spot will appear displaced left or right on screen 2 according to the bearing of the target left or right. For the present purpose, it is assumed that the target is directly ahead.

It is convenient to describe functionally the operation of some of the major components of the system of Fig. l., postponing the detailed description of the involved circuits.

Each trigger pulse from pulse generator i4 initiates the emission of a pulse of radio frequency energy from antenna l and at the saine time is supplied to actuate time base generator 24. Generator 2s produces a pair of voltage pulses oi opposite polarity and lasting for approximately 10) microseconds, which are both supplied to range sweep generator 5B, the negative pulse serving to f excite in generator 5E! a positive sweep voltage rising through a voltage range of about G volts linearly with time at a predetermined rate throughout the 100 rnicrosecond interval, the positive pulse producing a positive pedestal voltage on which is superposed the rising sweep voltage, This sweep voltage on a pedestal recurs with each radar emission and starts simultaneously therewith. t is supplied by range sweep generator El? at all times to range differential amplier Hf! and when switch S is closed upwards it is fractionally supplied also to vertical sweep amplifier 2m).

Rate sweep generator 30 produces a sweep volt ageslowly decreasing linearly with time from an Y 4 adjustable initial value and at an adjustable rate of decrease. This sweep voltageoccupies from 100 to 400 seconds to decrease through a range of 100 volts, so that throughout any 100 microsecond interval it may be considered constant. The output of generator d@ is likewise applied to range differential amplifier l lil. Obviously, the initial value of the decreasing .output voltage oi generator Sli may be chosen less than the maximum value reached by the rising voltage of genn erator E@ so that in each lo() microsecond intern val there will be an instant of equality oi the two voltages on the input of differential amplier lle. As the voltage from generator l slowly decreases this instant of equality will occur progressively nearer to the start of the lo() microsecond interval, that is to say, nearer to the moment Of emission of object ranging pulse from antenna l. Y

To anticipate the later description, it may here be said that the voltage from generator Si? is so chosen that at a given time the instant of equa-1- ity of the sweep voltages from generators d@ and 85B occurs simultaneously with the reception by antenna 'l of an echo reflected from a chosen target and the rate of decrease of the voltage from generator 8@ is so adjusted that this instant continues to occur simultaneously with the reiiected echo as the range of the target decreases. Clearly, the means which so sets the rate of voltage decrease affords a measure of the rate o i change of range of the target, that is to say, of the relative speed of target and plane. If the target is stationary and the planes altitude is not a large fraction of the plane to target dis tance,. the speed so measured is the ground speed of the airplane.

Before continuing the functional description oi the system of Fig. l it is proper here to describe the circuits so far involved.

eferring new to Fig. 2 a short positive trigger pulse from pulse generator if: is applied to grid of the tube V1, which is suitably SSTLE'Z, 'differentiation by the circuit comprising condenser 5 and resistance E?. Grid 225i tube V1 is negatively biased by battery 28 so that tube Vi is normally not conducting. Differcntiating circuitv CZSRZ'E produces a positive pip at the leading edge of the trigger pulse, an instant hereinafter designated as to. fi

trailing edge on the trigger pulse is disregarded. Prior to the arrival of the positive pip on grid 25 no anode current flows in tube V1 and there is no voltage drop across the resistor 29 through which anode 3B of V1 is connected to 300 Volt battery 3l. Battery 3| is also connected through resistor 32 to-anode 33 of tube V2, a double triode such as a 6N?, through resistor 3s to grid 35 and through resistor 29 to anode 35 of V2. Cathodes 38 and 39 are electrically connected together and through resistors l0 and di in series to ground. The junction of resistors lll) and 4l is connected to grid 42 through resistor i3 while grid d2 is shunted to ground by condenser dit. Cathode 45 of V1 is likewise grounded. n all circuits cathode heating power is understood to be supplied though not shown. Between ground and cathode 39 of V2 are connected condenser l5 and resistance il in series, from the junction of which, through condenser 4S shunted by resistor 59, a square topped voltage pulse negative to ground of itil microseconds duration is fed to range sweep generator 5U. Also to generator 5S a square topped voltage pulse, positive to ground, is fed from anode 3,3 oi V2. Of these voltage pulses, the former excites the rising sweep voltage produced by generator 50 while the latter 4provides the pedestal which the sweep voltage overlies.

In the circuit of Fig. 2, grid 25 of V1 is normally .biased to cut-off by battery 28. Grid 42 of tube V2 is biased to cut-off by the voltage developed across resistors 40 and 4| in series by the flow of current in the right half of V2 from anode 33 to cathode 38. Since grid 35 is connected through 1.5 megohm resistor 34 to battery 3|, its voltage is slightly higher than that of cathode 38, namely, about 20 volts positive to ground and the right half of V2 is normally conducting. Condenser 31 is connected between grid 35 and anode 36.

n A positive voltage pip drives grid 25 positive, so that V1 becomes conducting and its anode voltage falls. Anode 136 of V2 is connected directly to anode 30 of V1 and through condenser 31 to grid 35 of V2. The fall of voltage at anode 30 thus is coupled through condenser 31 to grid 35 to cut-off the right half of V2, and the consequent disappearance of current from resistors 48 and 4| permits the left half of V2 to become conducting.

Initially, V1 is not conducting, anodes 30 of V1 and 36 of V2 are 300 volts positive to ground. In V2 cathodes 38 and 3B as well as grid 35 are 20 volts positive while anode 33 is about 257 volts positive to ground, the right half of V2 being conducting while the left half of that tube is blocked. Grid 42 of V2 is thus 20 volts negative with respect to cathode 39 and condenser 31 is thus across a potential difference of 280 volts between anode 36 and grid 35. The positive voltage pip from differentiating circuit C26R21 makes V1 conducting and the potential at anodes 30 and 36 falls to about 165 volts. This drop of 135 volts at anode 36 is communicated through condenser 31 to grid 35 which accordingly falls to 115 volts negative to ground cutting oi the right half of V2 so that the potential of anode 33 rises to 300 volts. The current in resistors 40 and 4| becomes momentarily zero, thus removing the 20 volt negative bias on grid 42 so that the left half of V2 becomes conducting, its anode 36 remaining 165 volts positive to ground. A small current now ows in cathode resistors 40 and 4| and condenser 31 starts to readjust its charge to the new voltage difference about 146 volts, between anode 36 and grid 35. This involves a rise in potential of grid 35 which on reaching the cut-01T potential volts allows the right half of V2 to conduct. Now the flow of current of resistors 40 and 4| results in cut-01T of the left half of V2 and the initial conditions are restored. The readjustment of the charge of condenser 31 is by a partial discharge through resistor 34 and the left half of V2. The time constant C31R34 is 300 microseconds and the rise in potential at grid 35 of V2 from 1-115 volts to -10 volts requires 100 microseconds. During this interval the potential of anode 33 is 300 Volts rising abruptly from 267 volts at the instant V1 becomes conducting and falling rapidly 100 microseconds later. This furnishes a 33-volt positive square topped pulse. At the end of the 100 microsecond interval the potentia1 of anode 33 falls slightly below the initial value of 267 volts because of a small flow of current from grid 35 to cathode 38. The 33-volt positive pulse is used as pedestal voltage in range sweep generator 50 and the terminal distortion is unimportant. Condenser 44 of capacitance .006

microfarad holds grid 42 at constant voltage with respect to ground. Simultaneously with the positive pulse at anode 33, there is produced a negative, square topped, pulse across resistors 40 and 4| due to the abrupt drop and succeeding rise of current therein, a negative pulse which is taken olf between cathode 39 and ground and is used as above stated to produce the sweep voltage in generator 50. Here the terminal distortion is harmful and is removed by the lter circuit comprising condenser 46, resistor 4,1 and condenser 48 shunted by resistor 49.

The input terminals of the circuit of Fig. 2 are A and ground G, across which the trigger pulse from generator I4 is applied. The output terminals are B1, C1 and ground G1, the sweep producing pulse being taken between C1 and ground, the pedestal pulse between B1 and ground.

Time base generator 24, which the circuit of Fig. 2V Y,

constitutes, defines the duration of the voltage rise in range sweep generator 58 and thus the range of the most distant target to be considered. The microsecond interval, corresponding to a target distance of about 10 miles, is fixed by the choice of condenser 31 and resistor 34, in the case described 200 micromiorofarads and 1.5 megohms. respectively. The sweep interval is in any case preferably somewhat shorter than the interval between successive signals from antenna 1 which in some radar installations may be long enough for a 100 mile range to be dealt with.

In Fig. 3 is shown the circuit of range sweep generator 50. Input terminals for generator 50 are B2 and C2 on which are impressed positive and negative pulses from terminals B1 and C1 respectively, of Fig. 2, and ground G. The negative square topped voltage pulse at terminal C1. of Fig. 2 is applied at terminal C2 of Fig. 3 to grid 5| of tube Vs, a 6AC7, for example, initially conducting and rendered inactive when a negative pulse arrives at grid 5|. Screen grid 52 of V3 is supplied through resistor 55 from battery 3 i which may be the same as battery 3| serving to supply all voltages of the system of Fig. 1. Grid 52 is shunted to ground by condensery 56 while suppressor grid 53 and cathode 54 are grounded. Anode 51 is supplied through resistor 58 and bias control tube V5, a diode such as one-half of a 6H6, from the junction of resistors 59 and 60, these resistors constitute a voltage divider between battery 3| and ground whereby anode 6| of Vs is supplied with 50 volts. Cathode 62 of V5 is connected through resistor 58 to anode 51 of V2. Condenser 63 shunting resistor 53 is connected between anode 51 of V3 and grid 64 of tube V1 which is suitably one-half of a GSN'TGT. Anode 65 of V1 is supplied directly from battery 3| while between cathode 66 and ground are connected resistors 61 and 68 in series.

Resistor R, preferably 200,000 ohms, is connected between cathode 66 and the junction of condenser 63 with anode 51. Between anode 51 and input terminal B2 are connected condensei- C about 200 micromicrofarads, and condenser C', which may be 1,000 micromicrofarads, in series. Shunting this connection of condensers C and C are condensers 69 and 10 in series serving as a trimming capacitance. Condenser 69 is suitably an air condenser, while condenser 10 may have a capacitance of 1,000 micromicrofarads. Resistor R1. about 330,000 ohms, is inserted between cathode 66 and the junction of condensers C and C'.

It will be observed that the positive pedestal voltage pulses from time base generator 24 applied to input terminal B2 is interposed between ground and the circuit of Fig. 3 to the right of tube V3. Further, those acquainted with sweep voltage generators, well described, for example, in Time Bases by O. S. Puckle, published in London in 1943, will recognize that the circuit of Fig. 3 is such a generator, inactive while tube V3 is conducting but generating a rapidly rising voltage starting from the instant when V3 is blocked by the negative pulse applied to grid 5i from generator 24. .This rapidly rising voltage rises substantially linearly with time and continues so to rise until the negative pulse from generator 24 has passed from grid 5|. The rate of voltage rise, controlled by the ratio of the voltage across condenser 83 to the product RC, is in the present circuit about l volt per microsecond. This sweep voltage appears as a voltage positive to ground at cathode 60 to which output terminal D1 is connected. Tube V4 is an ampliiier tube supplying negative feedback to linearize this voltage wave as a function of time while the circuit R1C is an integrating circuit further contributing to the desired linearity.

The output voltage from the circuit of Fig. 3 is taken between terminal D1 and ground, or a desired fraction of it may be taken between terminal E1 and ground. Terminal D1 is used when switch S, Fig. l, is closed downward, terminal E1 when S is closed upward.

Resistors 55, 559 and are respectively about 68,000, 20,000 and 100,000 ohms while resistor 58 is 2.2 inegohrns. Resistors El' and 08 are about 250,000 and 50,000 ohms, respectively, so that the pedestal and sweep voltages at terminal E1 are each about one-sixth those at terminal D1.

It will be clear from the foregoing description that in the circuit of Fig. 2 tube V2 is a single- I.

shot multivibrator synchronized by tube V1 with the trigger pulse which simultaneously actuates radar system l. The output negative pulse from terminal C1 controls the conductance of tubel V3 in the circuit of Fig. 3, and the duration of the voltage rise at terminals D1 and E1 of Fig. 3. This voltage rise is linearized by negative feedback from tube V4 and further improved in linearity by the integrating circuit RC, for which values of resistance and capacity are chosen with. regard to the values of R and C and the amplification factor of tube V4. Diode V5 is so inserted that in the intervals between successive sweeps condenser 53, of .006 microfarad capacitance, may be rapidly charged by diode V5 through tube V3, which is during such intervals conducting, and so be at a fixed potential at the start of each successive pulse from tube V2. circuit of Fig. 3 is not itself a part of the present invention but is disclosed and claimed in the copending application of J lV. Rieke, filed March 21, 1044, Serial No. 527,457, assigned to the same assignee as the present application.

The voltage at terminal D varies from about 100 to about 200 volts, starting with about 65 volts during the interval between sweeps, to which a 433-volt pedestal is added at the start 0f the sweep.

The rate sweep generator, of which the circuit is shown in Fig. 4, provides a voltage slowly decreasing between terminal F1 and ground from about 200 to about 100 volts over a time interval varying from 11/2 to 6 minutes. The circuit of Fig. 4 includes vacuum tubes Vs, V1 and Vs and voltage regulator tube V9. Suitably tubes Ve and V7 are respectively, the two triodes contained in a GSL'T, Va is one-half of a 6SN7GT while V9 is a VR75. Battery 3| supplies the voltage required in the circuit of Fig. 4. Across this battery is connected potentiometer 8| of about The 10,000 ohms resistance, on which tap 82 selects a fractional voltage adjusted, as later described, to be proportional to the speed of the airplane relative to the target. This fractional voltage appears across resistor 93, about 1/2 megohm, and from a fixed point thereon about le of the voltage selected by tap 82 is applied through 3 megohm resistor 85 to grid 80 of tube V6. Cathode 81 is connected through resistor 88 to the positive terminal of battery 3| and to ground through the 300 ohms of resistors 89 and 90 in series. Variable resistor 89 is so adjusted that when tap 82 is at ground no current iiows in resistor 85.

Anode 9| of Vs is directly connected to cathode 92 of V7 of which grid 93 is positively biased from the junction of resistors 94 and 95 to a potential of about 45 volts. Anode 98 of V7 is supplied from battery 3| through lO-megohm resistor 91. Sweep condenser C, 4 microfarads, together with resistor 85 constitutes the sweep circuit controlled by the voltage taken between point 84 and ground. Effectively condenser C" is connected between grid 80 of Ve and anode 96 of V7, which tubes constitute a direct coupled direct current amplifier supplying negative feedback to linearize with time the variation in voltage across con-- denser C. Actually, instead of being directly joined to anode 98, condenser C" is connected to cathode 98 of tube V8, of which grid 99 is joined through resistor |00 to anode 98 of V7. Anode |02 of Vs is directly supplied from battery 3|, the load resistor of Vs being composed of voltage regulator tube V9 in series with resistor |03. Across tube V9 is shunted resistor |94 which may be of 100,000 ohms resistance and is tapped to furnish at terminal F1 a desired fraction of the constant voltage across tube V9, plus the decreasing voltage across resistor |03. Battery |05, derived from battery 3 provides a negative voltage to stabilize tube V9. Grid 99 of Vs is shunted to ground by condenser |08, which with resistor |00 serves to prevent oscillations of voltage at grid 99. Tube Vs functions as a cathode follower tube so that condenser C when connected between cathode 98 of Vs and grid 80 of V6 is effectively connected between that grid and anode 9S of V1. To increase the amplification positive feedback is provided by resistor |07 between cathode 98 of Vs and cathode 81 of Ve, thereby raising the amplification factor ofthe amplier circuit to 5,000.

Switch S is closed as shown in Fig. 4, when switch S of Fig. 1 is closed upward. Closing switch S connects battery 3| through 5,000 ohm resistor |08 to one plate of condenser C, the other plate thereof being connected to grid 86, which is at ground potential and only about 2 volts negative to cathode 8l. Condenser C accordingly charges to about 195 volts (battery |05 opposing battery 3|) positive to ground at cathode 98, through resistor |08 and the grid-cathode circuit of Ve. This voltage also appears across tube V9 and resistor |83, 75'Volts being across tube V9. Thus, the tap |09 on resistor |04 makes available at terminal F1 120 volts plus a desired fraction of '75 volts. This is a steady state Voltage independent of the operation of the sweep circuit of Fig. 3. The equality of this voltage with the sweep voltage from range sweep generator 50 can be set by adjustment of tap |09 to occur at any desired instant in the 100 microsecond interval between near its end and near its beginning.

When switch S is opened, condenser C" starts' discharge rate being controlled by the voltage at tap 84. From the stated values of capacity of condenser C and resistance of resistor 85 time constant CR85 appears to be 12 seconds, but the eifective time constant determining the linearity of the sweep is the product of this 12 seconds by the amplification factor obtained from tubes Va, Vv and V0, namely 1,000 minutes. In the circuit of Fig. 4 enough amplification is provided to make unnecessary an integrating circuit such as RC of Fig. 3. By analysis of the operation of Fig. 4 when switch S' is opened, it may be shown that as condenser C" discharges, grid 86 of V6 remains substantially at ground potential, so that the discharge current through resistor 85 is determined by the voltage at tap 84. The operation is in eect a cancellation of the charge placed on condenser C when S is closed, by an opposing sweep charge whereby the voltage across C is caused to fall at a rate equal to E/R85C volts per second where E is the voltage to ground at tap 84. When E is 12 volts the voltage at cathode 98 and so at terminal F1 will fall 1 volt per second, the voltage drop across V0 is constant. Therefore, if initially with S closed, tap |09 is at cathode 8 and E'=12 volts, the instant of equality of the voltages, from terminal F1 and from terminal D1 of Fig. 3 Will move when S is opened in 100 seconds from near the end to near the beginning of the 100 microsecond interval prescribed by time base generator 24.

The rate sweep circuit of Fig. 4 is also nota part of the present invention but is described and claimed in the copending application of J. W. Rieke above referred to.

In the system of Fig. 1, the major components following range sweep generator 53 and the rate sweep generator 8l] use known circuit arrangements and will be here described chiefly functionally, reference being made to the attached drawings for the circuit details. Referring to Fig. 5, vacuum tubes V10 and V11 of range differential amplier Hl receive on grids lll and H2, respectively, the voltages appearing at points D1 of Fig. 3 and F1 of Fig. 4. Of these voltages the first is a rising sweep voltage lasting 100 microseconds, the second is a voltage slowly decreasing over a comparatively long time equaled by the rising voltage at an instant in the 100- microsecond interval depending on the positions of taps 82 and m9 of Fig. 4. Tube V12 is an amplifying tube providing positive feedback to tube V10 through constant current tube V13 which is inserted between ground and joined cathodes H3 and H4 of tubes V10 and V12, respectively. The cathode current of tubes V10 and V12 is controlled by the potential of grid H of V13. Tube V11 is a buier tube protecting rate sweep generator 80 from loading due to grid current in tube V12, while voltage regulator tube V11 controls the screen voltage of V13.

It may be shown by analysis of the operation of the circuit of Fig. 5 that when the voltages at terminals D2 and F2 are equal there appears a square-topped positive pulse at anode H6 of V12 which continues to the end of the 1GO-microsecond intervall This pulse is supplied from terminal H1 to Video mixing amplifier |40 and from terminal H1 when switch S is closed downward to vertical sweep ampliiier 200.

The circuit of video amplifier Mii, of Fig. 1, is shown in Fig. 6. It comprises pulse amplifying tube V14, on grid 14H of which is impressed the pulse from terminal H1 of Fig. 5, and video amplifier tube V15 of which grid |42 receives at terminal K the echo signal from video ampliider l1 of Fig. 1. The bias of grid M2 is controlled by tube V17. The amplified positive pulse at anode E43 of V14 and the amplified echo signal at anode |44 of V15 are applied on grid M5 of tube Vis, from the cathode circuit of Which are fed a pair of negative voltage pips corresponding respectively to the arrival of the echo signal at terminal K and the start of the square-topped pulse applied to terminal H2. For a reason later given these voltage pips are delayed 5 microseconds by network 25). Ground terminals, not shown, are provided for the circuits of Figs. 5 and 6 and subsequent figures.

In Fig. 'Il is shown the circuit of final video amplifier l. Terminal L receives from terminal L of Fig. 6 the negative voltage pips, delayed 5 microseconds by network 250, and applies these to grid lll of tube V10 in amplifier H0. The amplification and reversal of sign of `these voltage pulses is accomplished by tubes V18 and V19 so that corresponding positive voltage pips are available at terminal N, To permit these voltlages to produce traces on screen 2 of oscilloscope 3 of Fig. l, the positive pips are superimposed on a positive pedestal voltage derived from tube V20 to grid 13 of which are applied via terminal N unblanking pulses that are explained in the description of Fig. 8. It is convenient to provide also at terminal Z a blanking voltage, derived in any convenient manner from radar system I to blank the oscilloscope trace during the rearward pointing of antenna l. This blanking voltage may be a positive voltage applied to grid i512 of tube V21 during such rearward pointing and re placed by a ground when antenna l points forward of the airplane. When present the blanking voltage annuls the output voltage at terminal N. Thus, only when antenna 1 points forward is the negative bias of grid i8, Fig 1, to be overcome and the trace is brightened only when a positive voltage pip appears at terminal N together with a pedestal voltage from tube V20.

In Fig. 8, the circuit of vertical sweep amplier 26) comprises tubes V22, V23, V24 and V25, Tubes V22 and V23 are suitably the two triodes of a GSN'IGT. Their respective anodes 2Q! and 2112 are supplied from battery 3l through resistor 23. Grids 23d and 265 are biased 50 volts negative by battery 295 through resistors 20T and Zi for grid 28d, 269 and 2li! for grid 205 and further biased by the voltage drop in common cathode resistor 2i l. When switch S, Fig. l, is closed upf ward the fraction of the output sweep voltage of range sweep generator 5d appearing at terminal E1 of Fig. 3 is applied via terminal I through condenser` 2l2 to grid 29d. dit the same time switch S ganged with switch S, is closed upward and grounds the junction of resistors 281 and 238 thereby removing from grid 266 the bias of battery 206. As a result, tube V22 becomes conducting, increasingly so as the sweep voltage rises at terminal I. A correspondingly increasing current flows in resistor 2i I, At the same time a negative voltage wave appears at anode 2i?! which is transferred from terminal M' to terminal M of Fig. 7. The bias on grids 2534 and 265 suces to cut off the pedestal of the voltage from the range sweep generator and only a rising voltage appears across resistor 2H to be transferred through stopping condenser 213 and resistor 2M to the junction of resistors 2| 6 and 2H of which the other terminals are connected respectively to grids 218 and 2| 9 of tubes V24 and V25. these grids being normally biased to cut off through resistor 220 by battery 22 l. Tubes V24 and V25 are amplifying tubes in parallel and at their anodes 222 and 223 there appears the amplified sweep voltage which produces a vertical ray deiiecting current in coil VDC of oscilloscope 3. A permanent magnet, not shown, is used to fix the starting point of the vertical sweep, preferably near the bottom of screen 2.

Referring again to video amplifier il@ of Fig. '7 the negative voltage wave arriving at terminal VM is reversed in sign in tube V20 and ultimately appears as an unblanking pulse across resistor H6 in the cathode circuit of tube V19. This pulse is applied to intensity grid i8 of oscilloscope 3 which thus allows the trace on screen 2 to brighten when there arrives a negative pulse at terminal L', Such a pulse, either a radar target echo or one occurring at the instant of equality of range sweep and rate sweep voltages produces a bright spot on screen 2 Referring to Fig. 8, when switches, S, S and S" are thrown downward, tube V23 is rendered conducting and the positive square-topped pulse produced by range diierential amplifier H produces a voltage across cathode resistor 2 ll which now is a square-topped wave, positive to ground, beginning at the instant of equality of range and rate sweep voltages and lasting to the end of the 100 microseconds time base interval. The cut- 01T bias of grids 2W and 2I9 is reduced to zero. The voltage at anodes 222 and 223 abruptly drops at the start of this cathode voltage and rises thereafter exponentially. This exponential rise in anode voltage of tubes V24 and V25 results in a rise in anode current which ows in coil HDC, the inductance of which is so chosen that the duration of this current is about 11 microseconds.

It is thus clear that when switches S, S and S are closed upward a vertical sweep starts from the bottom of oscilloscope screen 2 and lasts 100 microseconds. During this sweep a bright spot appears on screen 2 only when there arrives on grid i8 either a target echo from radar system l or a pulse from range diierential ampliiier l Hi at the instant range and rate sweep voltages are equal. The azimuth sweep current through coil HDC is controlled from potentiometer 22 of radar system i, so that a target echo brightens the oscilloscope trace at a point corresponding horizontally to the target bearing, vertically to the target range. On the other hand, the voltage equality pulseis independent of the rotation of antenna l and the corresponding trace brightness appears as a horizontal line.

Fig. 9A represents the appearance of screen 2 under these conditions. T is a target spot hori- Zontally centered while RL is a line formed by the fusion of spots representing equality of range and rate sweep voltages. The vertical position of spot T represents target range, decreasing as the plane iiies onward. Range line RL is made to intersect spot T at an initial instant by proper setting of tap H39 kof Fig. 4. Spot T appears lower and lower as time goes on. While tap m9 may be manually shifted to maintain coincidence of RL and T, it is convenient to throw downward switch S and switches S' and S ganged with it. Now, as previously described, a vertical sweep ll microseconds long starts only at the moment of equality of range and rate voltages and line RL appears in a fixed position on the screen, This position would be at the bottom were it not for delay network 25S which delays the echo pulse and the range line pulse each about microseconds.

Fig. 9B shows the appearance of screen 3 when switches S, S and S are thrown down-wards. Range line RL appears vertically centered and stationary on screen 3 since its creating voltage pip on grid le, although simultaneous with the equality of voltages starting the sweep, is delayed a constant 5 microseconds relative to the moment of such equality. This moment is continually earlier because the voltage at tap E32 on potentiometer Si determines the date of decrease of the rate sweep voltage, which accordingly equals the rising range sweep voltage at a continually earlier epoch in the time base interval. If tap t2 is so set that this advance of the moment of equality is proportional to the rate of decrease of the range from plane to target, spot T will continue to be intersected by line RL.

It will be noted that delay network 256 serves the purpose of Yplacing the intersecting range line and target spot on the screen in a position convenient for observation. Further, it will be realized that it is much simpler to adjust tap 82 to maintain the coincidence of spot T and line RL than it is by adjustment of tap H39 to follow the moving target spot of Fig. 9A.

The operative procedure thus requires that with switches S, S and S thrown upward, tap |89, Fig, e, be set to make line RL, Fig. 9A, intersect spot T. The ganged switches are thenthrown downward and tap 82 is adjusted so to control the rate of decrease of the rate sweep voltage, that in Fig. 9B line RL continues tointersect spot T. The setting of tap 62 thusproduces a voltage to ground proportional to the planes velocity relative to the target considered. This velocity, aspre-V viously stated, is with aA fixed target substantially the ground speed of the plane. VThis speed is thus measured by the system of the invention independently of the conditions of light or weather.

While Vthe invention has been described with reference to a situation in which the target range is continually decreasing, those skilled in the art are acquainted with means for inverting the decreasing voltage from the rate sweep generator to obtain an increasing voltage. This enables them to maintainV the range line RL intersecting target spot T as the range increases, thereby adapting the disclosed system to measure Vspeeds of recession as Well as of approach.

What is claimed is: Y

l. The method of measuring the speed of a vessel relative to an object at a varying range ahead of said vessel, said range being determinable, which comprises the steps of determining said range, defining a repetitive timeinterval, establishing a rst voltage rising substantially linearly with-time during said repetitive interval, estabvlishing a second voltage decreasing at a controllable rate substantially linearly with time over a desired interval longer than said repetitive interval, dening in said repetitive interval an instant representative of said range and varying in accordance therewith, adjusting said second voltage to equality with said iirst voltage at an instant in said repetitive interval coincident with said representative instant at an initial definition thereof, and controlling the rate of decrease of said second voltage to maintain the simultaneity of occurrence of said equality and said representative instant, whereby said rate of decrease s controlled is proportional to said speed.

2. Means for measuring the rate of decrease of the range from an airplane to a target ahead of said airplane comprising electrical means for ranging and locating said target including a cathode ray oscilloscope provided with a fluorescent screen on which the Vertical position of a 1uminous spot moves in accordance With the change in range of said target, electrical means for producing on said screen a luminous horizontal line intersecting said spot in an initial position thereof and electrical means including a potentiometer graduated in velocity units for moving said line to follow said spot.

3. Means for measuring the speed of a Vessel relative to an object ahead comprising` electrical means for ranging vand locating said object including a cathode ray oscilloscope provided with a fluorescent screen on which the vertical position of a luminous spot represents the range of said object, electrical means for producing a horizontal luminous line on said screen intersecting said spot in an initial position thereof and electrical means including a potentiometer graduated in velocity units for maintaining said spot and said line stationary on said scree 4. Means for measuring the Speed of a vessel relative to and in the direction of an observed target comprising electrical means for ranging and locating said target including a cathode ray oscilloscope provided with a fluorescent screen on which the vertical position of a luminous spot represents the range of said target, electrical means for producing a horizontal luminous line on said screen intersecting said spot in an initial position thereof and electrical means including a potentiometer graduated in velocity units for maintaining said line continuously intersecting said spot.

STEPHEN DOBA, JR. 

